The lunar distance method

In the 1750s, German astronomer and map-maker Tobias Mayer devised a lunar distance method for finding longitude at sea. Sailors used a sextant to measure the angle between the moon and a star to establish the time in Greenwich, England, and then compared it with the local time on board ship. This required very precise observations of the sky, only possible following the invention of the achromatic telescope.

English Astronomer Royal Nevil Maskelyne was a keen advocate of the lunar method over its rival claimant to the Board of Longitude's prize, John Harrison's marine chronometer. Mayer's widow received £3000 for her husband's work.

When James Cook sailed for the Pacific in 1768 he was carrying a nautical almanac containing tables showing the positions of celestial bodies for the following year. The tables had been feverishly prepared by Maskelyne's team of scientists to help navigators complete the time-consuming longitude calculations.

This diagram shows the angles and distances measured in the lunar method.
Courtesy: Based on an image by Michael Daly.

Captain James Cook used this sextant, made by Dollond, London, about 1770.
Courtesy: State Library of New South Wales